This invention is an improvement of U.S. Pat. No. 5,533,975 (hereinafter called xe2x80x9cprior artxe2x80x9d) also invented by the present inventor.
The prior art disclosed a safety syringe for automatically tilting a used needle when retracted into the syringe to prevent re-protrusion of the needle outwardly. A cylindrical recess 125 is recessed in the front portion of the syringe 1 provided with two wedged extensions 126 formed on the recess 125 for retarding two bars 225 diametrically formed on a cylindrical portion 224 of the shank portion 22 of the needle assembly, thereby preventing rotation of the shank portion when rotatably coupling the needle portion 21 and the shank portion 22. However, such a cylindrical recess 125 will be accumulated with liquid medicine to form a xe2x80x9cdead poolxe2x80x9d which can not be completely injected into a patient, causing waste of the medicine.
The present inventor has found the drawbacks of the prior art and invented the present improved safety syringe.
The object of the present invention is to provide a safety syringe including: a syringe cylinder having a plurality of sleeve ribs formed in a sleeve portion of the syringe cylinder, a needle device having a needle detachably coupled on an shank portion which is provided with a plurality of shank ribs on the shank portion to be engaged with the sleeve ribs in the sleeve portion, with the needle device retractable in and automatically biased in the syringe cylinder to prevent outward protruding of used needle; whereby upon external connecting and rotative coupling of the needle on the shank portion, the shank portion will be locked without being rotatable, thereby enhancing a smooth coupling of the needle on the shank portion of the needle device.
Another object of the present invention is to provide a safety syringe including an annular extension annularly formed on an inside wall of the syringe cylinder adjacent to a rear end of the syringe cylinder and at least a ratchet tooth formed on a front portion of a plunger slidably held in the syringe cylinder; whereby the annular extension provides a stopper for preventing a rearward releasing of the plunger from the cylinder since the ratchet tooth on the plunger will be locked against the annular extension when accidentally pulling the plunger rearwardly outwardly.